1. Field of the Invention
The present invention relates to a fibre-type light conversion device.
2. Description of Background Information
An optical pickup is known by which high density writing and reading of information on and from a disc are enabled by transforming the wave length of a laser beam emitted from a laser source to a half the original wavelength by means of a light conversion device (Japanese Patent Application Laid-Open No. 61-50122).
As the light conversion device for use in this pickup, there is an optical fibre type SHG (Second Harmonics Generator) using a second-order non-linear optical effect. A phase matching of Cerenkov radiation system is adopted in the optical fibre type SHG. With this system, it is possible to generate a second harmonic wave whose phase matching condition is attained almost automatically. The general conception of this device is shown in FIGS. 1A and 1B.
In FIG. 1A, when the fundamental wave mode is propagated through the core with the effective reflective index of N (.omega.), the non-linear polarizing wave generating the SH wave is also propagated at the same phase velocity C/N (.omega.) (C is the speed of light in a vacuum). It is assumed that this non-linear polarizing wave produces the SH wave in a direction making an angle .theta. with respect to the direction of the wave guide at a point A, and generates the SH wave in the direction of .theta. as before at a point B, after the elapse of a unit time. If the SH wave generated at the point A propagates through the clad and reaches to a point C after the elapse of a unit time and the angle .theta. is such an angle that lines AC and BC are perpendicular to each other, then the plane of the SH wave which is generated from the non-linear polarized wave becomes equal to BC, and as a result, a coherent SH wave is generated.
The condition of the phase matching is, according to the figure, as follows: EQU N(.omega.)=N.sub.clad (2.omega.) cos .theta. (1)
In other words, EQU N(.omega.)&lt;N.sub.clad (2.omega.) (2)
This means that the SH is generated automatically in the direction where the phase matching condition is performed when at least the condition mentioned by the equation (2) is satisfied. Generally, with the refractive indices of the clad and core with respect to the fundamental wave being n.sub.clad (.omega.) and n(.omega.), and with the air as the over-layer, the condition for the fundamental wave to propagate through the core as the mode is expressed as: EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n(.omega.) (3)
Wavelength dispersion of the clad's refractive index will now be considered. Since n.sub.clad (.omega.)&lt;n.sub.clad (2.omega.), if the equation (2) is satisfied for all of the fundamental wave modes irrespectively of the diameter of the core so far as the following expression (4) is satisfied. EQU N.sub.clad (.omega.)&lt;N(.omega.)&lt;n.sub.clad (2.omega.) (4)
Moreover, there are fundamental modes satisfying the equation (2) in a certain range of the diameter of core even under the following condition. EQU N.sub.clad (.omega.)&lt;n.sub.clad (2.omega.)&lt;n(.omega.)
The second harmonic wave generated in this way is propagated in a clad mode as illustrated in FIG. 1B in which total reflection occurs repeatedly at the boundary between the clad and air. Then, the second harmonic wave is emitted in conical shape from the end of fibre in directions making an angle .theta. relative to the fibre's direction. The equiphase front of the second harmonic wave emitted in this way is in a conical surface with an axis on the central axis of the fibre.
In order to efficiently apply this second harmonic wave in the opto-electronics, it is desirable to give the emitted wave surface a plane form. As illustrated in FIG. 2, it is conceivable to dispose a conical prism 20 with a conical surface having vertical angle of 2.alpha. in the optical path of the beam emitted from the wavelength conversion device 10, and the conical equiphase front can be converted to a planer equiphase front by collimating the second harmonic wave (making it parallel) by means of the function of the conical prism 20.
However, if it is attempted to make the conical prism 20 from a material such as plastics, there arises a problem that the so-called "shrinkage cavity" or "deformation" is generated during the moulding of plastics, because the thickness is largest at the central position of the prism in the case of the prism having a conical shape. Moreover, it is generally difficult to eliminate the "shrinkage" or "deformation" mentioned above.